1. Field of the Invention
The instant invention relates to chromatographic methods and apparatus for analyzing formation fluids and more particularly to such methods and apparatus wherein a downhole tool extracts a formation fluid sample in an uncased well bore.
2. Setting of the Invention
After drilling an oil well, it is desirable to evaluate the formation to determine whether or not sufficient oil or gas can be produced to justify the expense of completing the well with casing and production tubing.
One way to determine composition of reservoir fluids is to run a drill stem test. Such a test entails running a string of drill pipe having a packer suspended near the lower end thereof into the well bore. The bore is packed off just above the zone of interest and fluid from the formation is permitted to flow into the drill stem. The pressure of the flow is monitored and a sample is collected in the drill pipe. Thereafter, the drill string is raised and the sample is analyzed by injecting some of the sample fluid into a chromatograph. The drill stem test suffers from several drawbacks. First, it is expensive and time consuming to run in a string of drill pipe to the formation of interest. Secondly, when fluid samples are collected they tend to separate and must be recombined at the surface. A more accurate analysis could be obtained if the formation fluid could be analyzed prior to separation into gas and liquid phases. Also, if the well bore is in relatively soft rock a drill stem test cannot be run without casing the well bore thus adding a substantial additional expense to the cost of the test.
There are prior art formation testers which can be lowered into an uncased well bore on a wire line to a level adjacent a formation of interest. Thereafter, a sample pad is firmly engaged against the wall of the borehole and formation fluid is drawn through a conduit in the pad into the tool. Then, the tool is raised to the surface and the collected sample is analyzed in a chromatograph to determine the composition of the formation fluids. Formation testers are typically large and include either one large chamber or multiple smaller chambers for collecting samples at different levels in the well bore. A problem which exists with prior art formation testers is that the initial flow from the formation is filtrate which includes drilling mud particles and the like which become lodged in the formation adjacent the borehole during the drilling process. This presents a special problem when an oil based drilling mud is used since the presence of oil in the mud can show up in a chromatograph analysis and be mistaken for connate fluid from the formation. There is at least one prior art tester in which filtrate flow is received into a first chamber while resistivity of the flow is measured. When resistivity stabilizes, thus indicating presence of connate flow, the flow is diverted into a second chamber and the tester retrieved to the surface.
Prior art devices for collecting formation fluid samples are susceptible to collecting contaminated samples due to filtrate flow. Such devices are also limited in the total number of samples which can be taken without being retrieved to the surface. In addition such devices are typically quite long to accomodate multiple chambers. When a single sample at a single level is to be collected, the device is generally still long to accomodate a large chamber so that at least some of the flow into the chamber, following the initial filtrate flow, will comprise connate formation fluid. In addition to these drawbacks, by the time the fluid sample is retrieved to the surface and prepared for analysis in a chromatograph, it will have separated into its gas and liquid phases and must be recombined as in the case when a fluid sample is obtained via a drill stem test.
The method of the instant invention comprises the steps of lowering a chromatograph into a well bore to a level at which fluids of interest are contained in the formation adjacent the chromatograph. A fluid sample is extracted from the adjacent formation and is directed into the chromatograph which generates information relating to the composition of the sample. Such information is transmitted to the surface thus providing an immediate indication of the makeup of the fluid sample. In one aspect of the invention, formation fluid is withdrawn and the resistivity thereof is monitored until it stabilizes thereby assuring that a connate fluid sample is analyzed by the chromatograph. In another aspect of the invention, the chromatograph is purged with a solvent subsequent to testing at a selected level thereby permitting movement of the chromatograph to a different level for testing another fluid sample.
The apparatus of the invention performs the steps of the method.
The instant invention provides a method and apparatus in which a plurality of connate formation fluid samples may be taken at different selected levels in a well bore.
The instant invention provides such a method and apparatus in which the fluid samples may be analyzed by a downhole chromatograph and, in one aspect of the invention, the information generated thereby transmitted to the surface.
In another aspect of the invention, the chromatograph is purged with a solvent between each fluid sample analysis.